1. Field of the Invention
The present invention relates to a modulation circuit device used for a transmission circuit device such as a mobile phone and the like, and more in particular, it relates to a modulation circuit device for polar-modulating an input signal, a modulation method and a radio communication device using the same.
2. Related Art of the Invention
A quadrature modulator has been widely used for a modulation circuit device used for a transmission circuit device, which is used for the terminal of a mobile phone and a base station. In recent years, however, a polar modulation has been highlighted, which is a modulation form allowing data to be carried by a polar coordinate system (phase and amplitude) (for example, Japanese Patent Laid-Open No. 09-18451). The disclosure of Japanese Patent Laid-Open No. 09-18451 is incorporated herein by reference in its entirety.
In FIG. 31 is shown a conventional modulation circuit device 101 which performs such a polar modulation.
The modulation circuit device 101 consists of an angle modulator 102, a voltage control circuit 103, and an amplitude modulator 104.
The angle modulator 102 is a circuit which angle-modulates a carrier wave by a phase signal 106 inputted from a second input terminal 109.
The voltage control circuit 103 is used for supplying a stable voltage to the amplitude modulator 104, and is a circuit to amplify an amplitude signal 105 inputted from a first input terminal 108.
The amplitude modulator 104 is a circuit to amplitude-modulate the signal from the output of the angle modulator 102 by the signal of the output of the voltage control circuit 103.
In FIG. 8 there is shown an example of the amplitude modulator 104.
The amplitude modulator 104 consists of a bipolar transistor 111, a matching circuit 112, a bias circuit 113, and a matching circuit 114.
That is, one end of the matching circuit 112 is connected to the base of the bipolar transistor, and the other end of the matching circuit 112 is connected to the output of the angle modulator 102. Further, an emitter of the bipolar transistor 111 is grounded. Further, the one end of the bias circuit 113 and the one end of the matching circuit 114 are connected to the collector of the bipolar transistor 111. The other end of the bias circuit 111 is connected to the output of the voltage control circuit 103, and the other end of the matching circuit 114 is connected to an output terminal 110. Note that a bias circuit and a power supply connected to the base side of the bipolar transistor 111 are omitted from the illustration.
Next, the operation of a conventional modulation circuit device 101 will be described.
An amplitude signal 105 and a phase signal 106 from the data inputted to a data generator by an unillustrated data generator are generated. The amplitude signal 105 generated by the data generator is inputted to the firs input terminal 108. Further, the phase signal 106 generated by the data generator is inputted to the second input terminal 109.
On the other hand, a power supply voltage is supplied to the voltage control circuit 103 from a power supply terminal 107.
The amplitude signal 105 inputted to the first input terminal 108 is then inputted to the voltage control circuit 103 and it is amplified by the voltage control circuit 103, and after that, is outputted to the amplitude modulator 104. By using the voltage control circuit 103, a stable voltage can be supplied to the amplitude modulator 104. That is, a voltage change due to the change of an input impedance and the like of the amplitude modulator 104 can be avoided.
Further, the phase signal 106 inputted to the second input terminal 109 is inputted to the angle modulator 102. The angle modulator 102 angle-modulates the carrier wave by the phase signal 106 to be inputted. The signal angle-modulated become a signal of a constant envelope. The signal angle-modulated by the angle modulator 102 is inputted to the amplitude modulator 104.
The amplitude modulator 104 amplitude-modulates the signal angle-modulated by the angle modulator 102 by the signal of the output of the voltage control circuit 103. That is, the signal from the output of the voltage control circuit 103 is inputted to the collector of the bipolar transistor 111, which constitutes the amplitude modulator 104, through the bias circuit 113. Further, the signal of the output of the angle modulator 102 is inputted to the base of the bipolar transistor 111 through the matching circuit 112. An output signal is outputted from the collector of the bipolar transistor 111 through the matching circuit.
That is, by controlling the collector voltage of the bipolar transistor 111 by the signal of the output of the voltage control circuit 103, the amplitude modulator 104 outputs a signal treated with both of angle modulation and amplitude modulation. In this way, the signal of the output of the amplitude modulator 104 becomes a changing signal of the envelope. The signal of the output of the amplitude modulator 104 is outputted from the output terminal 110 as a polar modulated signal.
Note that, as the angle modulator 102, a phase modulator to perform phase modulation and the frequency modulator to perform frequency modulation can be used.
In FIG. 9(a) is shown a relationship between the second power of the supply voltage of the voltage control circuit 103 to the amplitude modulator 104 and the output power of the amplitude modulator 104. Further, in FIG. 9(b) is shown a relationship between the second power of the supply voltage from the voltage control circuit 103 to the amplitude modulator 104 and the phase shift of the amplitude modulator 104.
In FIG. 9(a), the abscissa shows the second power of the voltage supplied to the collector of the bipolar transistor 111 through the bias circuit 113, and the longitudinal axis shows the output voltage of the signal outputted from the matching circuit 114.
Further, in FIG. 9(b), the abscissa shows the second power of the voltage supplied to the collector of the bipolar transistor 111 through the bias circuit 113, and the longitudinal axis shows the phase shift of the signal outputted from the matching circuit 114.
In FIG. 9(a), in the range shown as a linear region, the relationship between the second power of the supply voltage and the output becomes a linear relationship. However, when the second power of the supply voltage becomes smaller than the linear range shown in FIG. 9(a), the relationship between the second power of the supply voltage and the output power becomes a non-linear relationship. Further, when the second power of the supply voltage becomes larger than the linear region shown in FIG. 9(a), the relationship between the second power of the supply voltage and the output power becomes the non-linear relationship.
Further, in FIG. 9(b), in the range shown as a flat region, the phase shift is constant for the second power of the supply voltage. However, in the case where the second power of the supply voltage is smaller than the flat region shown in FIG. 9(b), when the second power of the supply voltage changes, the phase shift also changes. Further, in the case where the second power of the supply voltage is larger than the flat region shown in FIG. 9(b), when the second power of the supply voltage changes, the phase shift also changes.
The reason why such a relationship of FIG. 9(a) is obtained can be considered as follows. That is, the voltage from the output of the voltage control circuit 103 is applied to the collector of the bipolar transistor 111 of the amplitude modulator 104, and when this voltage becomes low, the voltage of the collector of the bipolar transistor 111 comes close to the voltage of the base of the bipolar transistor 111. Consequently, the linearity disappears. In other words, when the supply voltage becomes small, as shown in FIG. 9(a), the relationship between the second power of the supply voltage and the output power becomes a non-linear relationship. Further, when the voltage of the collector of the bipolar transistor 111 becomes high, the linearity disappears due to saturation and heat dissipation. Consequently, when the supply voltage becomes high, as shown in FIG. 9(a), the relationship between the second power of the supply voltage and the output voltage becomes the non-linear relationship.
Meanwhile, as for the signal from the output of the amplitude modulator used in the amplitude circuit device to perform the polar modulation, a signal sufficiently deeply amplitude-modulated is desired. In order to sufficiently deeply amplitude-modulate, it is necessary to sufficiently deeply enlarge a dynamic range of the supply voltage supplied from the voltage control circuit. However, when the dynamic range is enlarged, the second power of the supply voltage is forced out from the range of the linear region of FIG. 9(a) and the flat region of FIG. 9(b), and the signal outputted from the amplitude modulator ends up being distorted.
Consequently, in order to obtain the signal sufficiently deeply amplitude-modulated by such an amplitude-modulator, it is necessary to use a transistor which is sufficiently wide in the range shown as the linear region of FIG. 9(a). Further, it is necessary to use a transistor which is sufficiently wide in the range shown as the flat region of FIG. 9(b). This is difficult to realize by a single transistor, and moreover, in the case where this is realized by using a plurality of transistors, it becomes complicated to control these transistors, and moreover, in the case where distortion correction is performed by a table, the amount of memory required becomes large.
That is, in the case where the transistor used in the amplitude modulator of the modulator for performing the conventional polar modulation is a transistor not having the linearity in a wide range, there is a problem in that a signal sufficiently deeply amplitude-modulated cannot be obtained.
Further, when an attempt is made to obtain the signal sufficiently deeply amplitude-modulated by the conventional modulator for performing the polar modulation, a transistor having the linearity in a wide range is required. This is difficult to realize by a single transistor, and moreover, in the case where this is realized by using a plurality of transistors, it becomes complicated to control these transistors, and moreover, in the case where distortion correction is performed by a table, there is a problem in that the amount of memory required becomes large.
Considering the above-described problems, it is an object of the present invention to provide a modulation circuit device, a modulation method and a radio communication device, which can obtain a desired signal even when the transistor used as the amplitude modulator of the modulator for performing a polar modulation is a transistor not having a linearity in the wide range.
Further, considering the above-described problems, it is an object of the present invention to provide a modulation circuit device, a modulation method and a radio communication device, which can obtain a desired signal by using a single transistor by the modulator for performing a polar modulation or without becoming complicated when a plurality of transistor is used.